The present invention relates to a boost compensator for controlling the position of a control rack of a fuel injection pump to supply fuel to an internal combustion engine with a supercharger in response to a boost pressure to be applied to the engine, i.e., in response to an amount of intake air to be supercharged into the engine.
A boost compensator of the kind referred to above is known, for example, from Japanese Provisional Utility Model Publication (Kokai) No. 60-107337. According to the known boost compensator, as shown in FIG. 1 of the accompanying drawings, a push rod 7 is disposed for forward and rearward movement in response to a boost pressure to be applied to an internal combustion engine with a supercharger and supplied to a boost pressure chamber 3. The push rod 7 has one end thereof connected to a diaphragm 5 which is urged by a boost compensator spring 6. A main lever 10 supported for angular movement about its fixed fulcrum 11 has one end thereof engageable with an abutment 9 mounted on the push rod 7 adjacent the other end thereof. A control rack 13 interlocked with a governor of a fuel injection pump, not shown, abuts against the other end of the main lever 10 and is adapted to remain at a position where the urging force of the boost compensator spring is balanced with the urging force of a control spring, not shown, and the like associated with the governor.
With the arrangement of the conventional boost compensator described above, as shown in FIG. 2 of the accompanying drawings, as the boost pressure exceeding a first set value P.sub.1 as shown in FIG. 5 is transmitted to the diaphragm 5, the push rod 7 is moved forwardly by the diaphragm 5 against the urging force of the boost compensator spring 6. This permits the main lever 10 to angularly move, so that the control rack 13 moves by a predetermined distance or a fuel supercharging stroke M.sub.1 from a first position L.sub.1 to a second position L.sub.2 in a direction increasing the amount of fuel to be supplied to the engine, until the boost pressure reaches a predetermined value P.sub.2.
In the above-described conventional boost compensator, however, the control of the position of the control rack 13 by means of the main lever 10 is merely effected with respect to the first and second set values P.sub.1 and P.sub.2 of the boost pressure, and the control rack 13 remains at the predetermined position L.sub.2 so long as the boost pressure exceeds the second set value P.sub.2. Consequently, the amount of fuel to be supplied to the engine continues to increase as the boost pressure increases due to the operation of the supercharger, ultimately leading to a possibility that a bad influence is exerted upon the engine.
Conventionally, means is provided for reducing the boost pressure by a relief valve, not shown, when the boost pressure exceeds a predetermined limit. According to this means, however, it is required to arrange a detecting mechanism for actuating the relief valve and other requisite mechanisms. Accordingly, the engine system is complicated in construction, and the cost is increased.